FIELD OF THE INVENTION
The present invention concerns a method of operating thyristor type switches to provide zero voltage switching power supply to a three-phase power circuit, in particular a circuit in which the power is transmitted through a transformer, the method being remarkable in that it combines the highest possible performance (complete cycles with no steep fronts and no short cycles) with an economical power circuit comprising only two switches each connected to a respective phase, the third phase being connected direct to the load circuit.
Over the last two decades zero voltage switching of thyristors has found increasing application in the control of heating in industrial processes such as the manufacture and treatment of glass. The power is varied by modulating the duty cycle, that is to say by varying the duration of the series of cycles relative to the times between series of cycles.
Zero voltage switching control of thyristors has two advantages over the conventional "phase adjustment" technique:
1) Steep fronts on the voltage and current waveforms are eliminated from the AC line voltage and from the load circuit.
2) The power factor is improved.
Steep fronts on voltage and current waveforms generate interference and are likely to disrupt the operation on other electronic circuits, both analog and digital, supplied by the same AC line voltage. Also, it is imperative to maintain the power factor at a value approximating unity when the power concerned is not negligible in comparison with the total power absorbed. Unlike phase control, zero voltage switching does not introduce any phase shift between current and voltage and also makes it possible to equalize the overall power absorbed by multiple heating channels.
The presence of an electromagnetic transformer as an intermediary element in the transmission of the power delivered by the thyristor switches connected to the primary and the load circuit connected to the secondary means that the method of operating the thyristors must conform to the magnetic saturation limits of the transformer, because of the transient conditions associated with the start and end of the series of cycles. This results either in increased complexity of the equipment or reduced performance and zero voltage switching methods for supplying power three-phase circuits through transformers are currently of two kinds:
1) High-performance methods of the kind described in published French patent application No. 2 122 022.
In these methods the potential performance capabilities of zero voltage switching are fully exploited. The voltage and current waveforms are free of any steep fronts and the duration of the working cycles is minimal with the result that fluctuations in the power delivered to either side of its mean value are minimal whether for a single heating channel or multiple heating channels, enabling optimum equalization of the overall power. On the other hand, these methods use three switches, one for each phase, and are therefore less economic.
2) Methods which use only two switches for the three phases, of the kind described in published French patent application No. 2 089 138. These methods are economic but offer poorer performance because they introduce steep voltage and current fronts at the start of each series of cycles and because they can operate only with long working cycles in order to provide periods between two series of cycles to allow sufficient magnetic relaxation of the transformer.
An object of the present invention is to propose a method which represents a synthesis of the previous methods, integrating the high performance of the former method with the latter type of device comprising only two switches.